Lesson 6 Design Load Calculations : Part I

6.1 INTRODUCTION

Load and design requirements for the design of greenhouse structures, their components and enclosure elements (cladding) are as below. The loads specified herein are based on the ASCE 7. The loads are to be used in conjunction with the stress criteria of the International Building Code and referenced standards. Where no standards are referenced in the building code, recognized manufacturer’s literature may be used with regard to code compliance.

6.2 LOADS

  1. Dead and Live Loads - defined by the building code

  2. Environmental Loads - defined by the building code

  3. Collateral Loads - weight of support equipment used for the operation or maintenance of plant material, including water.

  4. Plant Live Load - weight of supported or suspended plant material  

Importance Factors: Iw  (wind), Is (snow), and I (seismic) - a factor that accounts for the degree of hazard to human life and damage to property.

6.3 BASIC REQUIREMENTS

6.3.1 Design

Greenhouse structures and all parts thereof shall be designed and constructed to safely support all loads.  These loads include the dead and live, collateral loads, environmental loads and equipment loads specified by the purchaser.

6.3.2 Serviceability

Greenhouse structures and their components shall have adequate stiffness to limit vertical and transverse deflections, vibrations or any other deformation that may adversely affect their serviceability. 

Dead and live load deflection shall not exceed the deflection limits specified in the building code. Table 1604.3 of the IBC gives vertical deflection limits as l/120.  While there are drift limits in the code for seismic design (IBC, Section 1617.3), lateral displacements are not regulated by the code for wind.  

However, even when wind loads govern the design of a building, the lateral force resisting systems shall meet seismic detailing requirements and limitations. 

Cladding attachment must be designed to accept differential movement under loads. 

6.3.3 Analysis

The design of greenhouse structures, the load effect on the individual components and connections shall be determined by rational engineering analysis methods. Rational engineering analysis is a computational analysis, either by hand or computer, that uses accepted load distribution and determination methods.   Unusual structural and construction methods shall be based on engineering analysis or physical testing by an approved laboratory. Greenhouse structures shall be analysed for all building code required load conditions. Elements and components shall be designed for load combinations specified in the building code or referenced standards. 

6.4  ADMINISTRATIVE ISSUES

6.4.1  Design Requirements

Prior to design the manufacturer should obtain local load information, i.e. wind, snow, etc. Information should include:

  •  Code of jurisdiction

  •  Determination of loads:

  • Roof Live Load

  • Wind speed  (3-second gust wind speeds)

  • Snow load  (ground snow load)

  • Earthquake zone or design spectra 

  •  Soil type and allowable pressure 

6.4.2 Required Information on Plans

Certain information must be shown on the construction drawings. The following information shown below is required even if it is not a controlling design load.  Information to be provided on the plans includes:

  • Dead Loads

  • Roof Live Loads

  • Collateral Loads (irrigation equipment, including water)

  • Plant loads

  • Snow Loads

  • Ground Snow Load pg

  • Flat-roof snow load, pf

  • Snow exposure factor, Ce

  • Snow load importance factor, Is

  • Thermal factor, Ct

  • Wind Load
  • Basic wind speed (3 second gust), miles per hour

  • Wind importance factor, Iw and building category

  • Wind exposure category

  • Applicable internal pressure coefficient and prevailing wind direction

  • Design Wind Pressure on Components and cladding.  

  • Exterior components and cladding materials are not specifically designed by the design professional.

  • Earthquake design data
  • Seismic use group

  • Spectral response coefficients (SDS and SD1)

  • Site class

  • Basis seismic-force resisting system

  • Design base shear

  • Analysis procedure

  • Flood load –

If a building is located in a flood hazard area, established by a jurisdiction having authority, the following shall be shown for areas not subject to high-velocity wave action:

  • Elevation of the lowest floor

  • Elevation to which any non-residential building will be dry flood proofed 

  • Foundation Design

Reactions of structural elements when the foundation or other systems are to be designed by others. If the structure is designed for future additions, the foundation information should include the probable design load information.

6.4.3 Additions and alterations

Additions to existing greenhouses may be made. The new structure shall not make the existing structure unsafe. For structural purposes it is related to the per cent of overstress in structural members. When a greenhouse is added to an existing building, the capability of the building to withstand any loads superimposed by the greenhouse shall be verified including lateral loads due to attachment and snow drift loads due to proximity. Alterations may be made to any greenhouse if any loads imposed on the existing structure do not create an unsafe condition. 

6.4.4  Load Testing

Load testing is typically not desirable for any product that is within the scope of computational analysis. Typically, specialty products such as cladding components are candidates for testing rather than calculations. Any load testing must be carried out by an independent approved testing agency.

6.5 DESIGN METHODOLOGY

6.5.1 Allowable stress design vs strength design requirements - Design of typical greenhouse structures may be made by using the allowable stress design (ASD) or the strength (LRFD) design methods. The load combination equations used will depend on the design method. The ASD is the most common approach used by most engineers for greenhouse structures. 

6.5.2 Safety factors for greenhouse components - Safety factors for the structural members are included in the code referenced standards. 

6.5.3 Greenhouse classification (Code occupancy group under IBC 2000) – Greenhouse structures may be considered an occupancy classification “U” when used as a Production Greenhouse.  Research facilities may be considered the same. Commercial greenhouse structures used for retail use are considered as a “B” or “M” occupancy classification. This is based on the fact that the building is normally occupied. 

6.5.4 Deflection and Drift - There is no criteria limiting drift. The engineer should consider the serviceability requirements of the building, previously discussed in Section 6.3.2.

6.6  LOADS

6.6.1 General  

Buildings and other structures shall be designed to resist the load combinations. Applicable loads shall be considered, including both earthquake and wind, in accordance with the specified load combinations. Effects from one or more transient loads not acting shall be investigated. 

6.6.2  Dead loads

  • Structure weight

  • Cladding weight

6.6.3  Live loads 

6.6.3.1  Roof

  • 10 psf minimum in the IBC (ASCE -7 permits the Authority having jurisdiction to accept 10 psf.)

6.6.4  Collateral Loads 

Collateral loads shall not be included in Wind Uplift resistance analysis. These loads shall be considered a live load for wind design. 

  • Mechanical Equipment - Irrigation, transfer systems, etc.

  • Permanently mounted service equipment (heaters, fans, water lines, etc.)

Such permanently mounted equipment shall be considered as a dead load when considering load combinations.

6.6.5 Plant Loads

Hanging plants, 2 psf minimum, applied as a concentrated load at the truss panel points.  Greenhouse purchasers may have additional or other criteria for hanging plant loads or mechanical watering systems. 

6.7  SNOW

6.7.1  General

Provisions for the determination of design snow loads on greenhouse structures are as per the ASCE 7 98(Section 7.0). They apply to the calculation of snow loads for both continuously heated greenhouses and for intermittently heated or unheated greenhouses. 

6.7.2  Definitions

The following definitions apply only to this section.

  • Continuously heated greenhouse. Any greenhouse, production or commercial, with a constantly maintained interior temperature of 50°F or more during winter months. Such a greenhouse must also have a maintenance attendant on duty at all times or a temperature alarm system to provide warning in the event of a heating system failure. In addition, the greenhouse roof material must have a thermal resistance (R -value) less than 2.0 ft2·hr·°f/Btu.

  • Intermittently heated or unheated greenhouse. Any greenhouse that does not meet the definition of a continuously heated greenhouse. 

6.7.3 Design Procedure

The elements outlined herein are the general process for snow design. Design snow loads for greenhouses shall consider following factors;

  • The ground snow load pg - based on map in code or local requirements

  • The flat -roof snow load pf calculated taking into consideration the roof exposure, the roof thermal condition, and the occupancy of the structure. 

  • The sloped-roof snow load ps for greenhouses with gabled, hipped, arched, and gutter connected roofs shall be determined as referenced in 6.7.4

  • Partial loading conditions to account for wind scour, melting, or snow –removal operations shall be considered as referenced in 6.7.4. 

  • Unbalanced snow loads due to the effects of winds on sloped roofs shall be considered as referenced in 6.7.4. 

  • Local snow load surcharges due to snow drifts on lower roofs and from roof projections as referenced in 6.7.4. 

  • Local snow load surcharges from snow sliding off of adjacent higher sloped roofs shall be considered as referenced in 6.7.4. 

6.7.4 Calculation of Snow Loads

6.7.4.1 Ground Snow Loads: As per ASCE 7 Section 7.0, or local code requirements.

6.7.4.2 Flat-Roof Snow Loads: (ASCE 7, Equation 7-1) Although greenhouses rarely, if ever, have flat roofs, the calculation of flat -roof snow loads, pf, is necessary for the calculation of sloped-roof snow loads, ps.

A flat roof is a roof with a slope less than or equal to 5 degrees. For low -sloped roofs refer to ASCE 7, Section 7.3.4 for further information and load limitations.

First the flat roof snow load pf is calculated.  If the building has a low-slope roof generally between 5 and 15 degrees), the flat roof snow load will have a minimum value determined by the Code.  The governing flat roof snow load, either calculated or Code-determined minimum, is then used to determine the sloped roof snow load, ps by multiplying with a slope factor Cs.

If the building has a sloped roof (greater than 15 degrees), the calculated value for pf  is used, with a slope factor Cs, to determine the sloped roof snow load ps. . For greenhouses, where the ground snow load, pg, is in the 15 psf to 20 psf range, the snow load will generally govern over the roof live load.  

For gutter-connected greenhouses resulting in a multiple folded plate, saw-tooth or barrel vault roof, the value of Cs is 1.0.

The flat roof snow load pf shall be calculated using the following equation, with exposure factor Ce, thermal factor Ct  and snow importance factor Is found in ASCE 7. 

pf = 0.7* Ce *. Ct. * Is * pg

The flat roof snow load pf, for low-sloped roofs only, shall not be less than the following:

pf = IPg ,when  pg is less than or equal to 20 psf  or

pf = Is 20 psf when pg is greater than  20 psf

Where,

Pg = Ground snow load, as per ASCE 7, Figure 7-1

Ce = Exposure factor, as per ASCE 7, Table 7 -2

Ct = Thermal factor, as per ASCE 7, Table 7 -3

Is = Importance factor for snow loading, as per ASCE 7, Table 7 -4 

Exposure Factor:   is a function of the greenhouse site terrain category and roof exposure category.  Most greenhouse roofs are likely to be fully or partially exposed and located in Exposures B or C. Thus, the snow exposure factor is most likely to be 0.9 or 1.0. 

Thermal Factor:  is a function of the thermal resistance of the greenhouse roof glazing and the temperature conditions within the greenhouse, and shall be determined from the following Table: 

Table 6.1 Thermal Factor, Ct

Thermal condition

Ct

Continuously heated greenhouse (see 6.7.2)

0.85

Intermittently heated greenhouse  kept just above freezing

1.1

Unheated greenhouse

1.2

All greenhouses except those above 

1.0

Note:

The thermal condition should be representative of the anticipated conditions during winters or the life of the greenhouse.

Snow Load Importance Factor: The value of the snow load importance factor, Is, used in the calculation of pf is a function of the type of greenhouse and its use, and shall be determined in accordance with the following Table: 

Table 6.2- Classification of Greenhouses for Snow Load Importance Factors

Category

ASCE7  IBC    

Nature of occupancy and location of Greenhouse

Factor Is

 II              I

All commercial greenhouses that are not in ASCE 7 Category I (IBC Category IV)

1.0

 I              IV

Production greenhouses that are occupied for growing plants on production or research basis, without public access 

0.8

6.7.4.3 Sloped-Roof Snow Loads: (ASCE 7 Section 7.4) The sloped -roof snow load, ps , shall be obtained by multiplying the flat -roof snow load, pf, by the roof slope factor, Cs .

Warm-Roof (Ct £ 1.0) Slope Factor, Cs: For all greenhouses, except unheated and intermittently heated greenhouses kept just above freezing with unobstructed slippery roof surface that will allow snow to slide off the eaves (such as light transmitting coverings including plastics, glass and  similar materials), the roof slope factor shall be determined by using the following formula, as depicted in ASCE 7, Fig. 7-2a:

Cs = 1 – [(θ-5)/65]                         (when θ > 5°)

Where, θ is the angle of slope from the horizontal in degrees. 

Warm-roof slope factors for common roof slopes are given in the following Table:

Table 6.3 - Common Warm-roof Slope Factors

Roof slope

Cs

3/12

0.85

4/12

0.80

6/12

0.65

8/12

0.55

12/12

0.40

Gutter connected

1.0

Greenhouses Kept Just Above Freezing (Ct = 1.1) Roof Slope Factor, Cs: For all intermittently heated greenhouses kept just above freezing with unobstructed slippery roof surface that will allow snow to slide off the eaves (such as light transmitting coverings including plastics, glass and similar materials) the roof slope factor shall be determined from the average of the values obtained for warm-roof slope factors and cold -roof slope factors. For common roof slopes these values are given in the following Table: 

Table 6.4 - Common Roof Slope Factors Cs for Just Above Freezing Greenhouse

Roof slope

Cs

3/12

0.95

4/12

0.90

6/12

0.80

8/12

0.60

12/12

0.45

Unheated Greenhouse (Ct = 1.2) Roof Slope Factor, C: For all unheated greenhouses with unobstructed slippery roof surface that will allow snow to slide off the eaves (such as light transmitting coverings including plastics, glass an similar materials), the roof slope factor shall be determined by using the following formula, as depicted in ASCE 7, Fig. 7 -2b: 

Cs = 1 – [(θ-15)/55]

Where, θ is the angle of slope from the horizontal in degrees.

Unheated greenhouse roof slope factors for common roof slopes are given in the following Table: 

Table 6.5 - Common Unheated Roof Slope Factors

Roof slope

Cs

3/12

1.00

4/12

0.95

6/12

0.75

8/12

0.65

12/12

0.45

Gutter connected

1.00

Curved Roof Slope Factor, Cs: (ASCE 7, Section 7.4.3) Portions of arched greenhouse roofs having a slope exceeding 70 degrees shall be considered free of snow load (i.e. Cs = 0). The point at which the slope exceeds 70 degrees shall be considered the “eave” for such roofs. For arched roofs the roof slope factor shall be determined from the appropriate formula in Sections 6.7.4.3, by basing the angle of slope on the slope line from the “eave” to the crown. 

Multiple Roofs Slope Factor, C, (Gutter-Connected): (ASCE 7, Section 7.4.4) Gutter connected (multiple) gable, saw tooth and barrel vault greenhouse roofs shall have a Cs = 1, with no reduction in snow load because of slope (i.e., ps = pf).  Greenhouse design should consider future additions when the gutter is on an exterior wall or on a single building to allow for future additions.

Ice Dams and Icicles along Eaves:  (ASCE 7, Section 7.4.5) Two types of warm roofs that drain water over their eaves shall be capable of sustaining a uniformly distributed load of 2pf   on all overhanging portions.  These roof types include the unventilated roof with an R - value less than 30 ft2.h.°F/ BTU, and the ventilated roof with an R -value less than 20 ft2.h.° F/ BTU.  No other loads except dead loads shall be present on the roof when this uniformly distributed load is applied.

6.7.4.4 Partial Loading: (ASCE 7, Section 7.5)  Roofs with continuous beam systems need to be designed for the partial loading of selected spans with the balanced snow load, while the remaining spans are loaded with half the balanced snow load. 

6.7.4.5 Unbalanced Snow Loads: (ASCE 7, Section 7.6) The combination of snow and wind from all directions contributes to unbalanced snow load conditions.  The amount of the unbalanced snow load is often dependent upon the width of the building, as well as the slope of the roof.  The gable roof drift parameter b, based on the relative shape of the building, and the snow density g, derived from the ground snow load, are used to determine the slope of the roof that limits the amount of unbalanced snow loads for the varying roof shapes.

6.7.4.6  Drifts on Lower Roofs (Aerodynamic Shade):  (ASCE 7, Section 7.7) Greenhouse roofs shall be designed  to sustain localized loads from snow drifts that form in the wind shadow of higher portions of the same structure and adjacent structures and terrain features. 

Lower Roof of a Greenhouse: (ASCE 7, Section 7.7.1) Drift loads shall be superimposed on the balanced snow load.  As the difference in adjacent building heights approaches zero, drift loads are not required to be applied.  Refer to ASCE 7 for surcharge loads from leeward drifts, formed by snow coming from a higher upwind roof, and windward drifts, formed next to a taller downwind building.

Note that the clear height difference between the upper roof height and the top of the balanced snow load on the lower roof, hc, is determined based on the assumption that the upper roof is blown clear of snow in the vicinity of the drift. This is a reasonable assumption when the upper roof is nearly flat. However, sloped roofs often accumulate snow at eaves. For such roofs, it is appropriate to assume that snow at the upper roof edge effectively increases the height difference between adjacent roofs, and using half the depth of the unbalanced snow load in the calculation of hc produces more realistic estimates of drift loads.

Adjacent Structures and Terrain Features:  (ASCE 7, Section 7.7.2) The effect of higher structures or terrain features within 20 feet of a lower roof shall be considered in the design of that lower-roofed building.

6.7.4.7 Roof Projections: (ASCE 7, Section 7.8) Gives a method that shall be used to calculate drift loads on all sides of roof projections and at parapet walls.  If the side of a roof projection is less than 15 ft. long, a drift load is not required to be applied to that side. 

6.7.4.8 Sliding Snow: The extra load caused by snow sliding off a sloped roof of a greenhouse or other structure onto a lower greenhouse roof shall be superimposed on the balanced snow load. It shall be determined assuming that all the snow that accumulates on the upper roof under the balanced loading condition (p times the roof area) slides onto the lower roof. Even if the upper roof is a greenhouse roof that is an unobstructed slippery surface, it shall be considered as not being slippery for purposes of calculating the extra sliding snow load. The final resting place of snow that slides off a higher roof onto a lower roof will depend on the size, position and orientation of each roof. Distribution of the sliding snow might vary from a uniform load 5 feet wide if a significant vertical offset exists between the two roofs, to a 20 foot wide uniform load where a low slope upper roof slides its load onto a roof that is only a few feet lower or when snow drifts on the lower roof create a sloped surf ace that promotes lateral movement of the sliding snow. 

6.7.4.9  Rain-on-Snow Surcharge Load: Rain-on-snow surcharge loads need not be considered on greenhouse roofs when they have slopes that exceed ½ inch per foot. However, all gutters in gutter-connected greenhouses shall be provided with adequate slope and drains to allow for run off of rain and snow melting and to prevent ponding. 

6.7.4.10 Ponding Instability: Roofs shall be designed to preclude ponding instability.  For roofs with a slope less than ¼  in./ ft., roof deflections caused by full snow loads shall be investigated for ponding instability from rain-on-snow or from snow meltwalter. 

6.7.4.11  Existing Roofs: Existing roofs shall be evaluated for increased snow loads caused by additions, alterations, and new structures located nearby, and strengthened as necessary. 

References:

1. www.ngma.com : Design Considerations, Chapter 2-7.

Last modified: Monday, 16 December 2013, 7:21 AM